Metal-Organic Frameworks and the structuring into nanofibers for gas adsorption

Zeyu Li

Research output: Book/ReportPh.D. thesis

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Abstract

Biogas is a promising alternative fuel to replace natural gas in a fossil-free energy system in the future. Biogas is a gas mixture of biomethane (typically 45-75 vol.% CH4), carbon dioxide (CO2), and small quantities of other gases (e.g., H2S). By using adsorption technologies to remove CO2 and other impurities, biogas can be purified and upgraded to biomethane (>98 vol.% CH4), resulting in a high calorific value of approximately 36MJ/m3. The upgraded biomethane has then the required purity (and specifications), so it can be directly used as fuel in the existing infrastructure (gas grid) or as fuel for use in compressed natural gas (CNG) or adsorbed natural gas (ANG) technology. For the commercialization of ANG in transportation, highly porous adsorbent materials, such as activated carbons, zeolites, or metal-organic frameworks (MOFs), are needed that fulfill the specifications (e.g., gravimetric and volumetric storage capacities, filling time, and durability), as defined by the U.S. Department of Energy (DoE). The same type of adsorbents can also be tailored and used for the upgrading of the biogas in swing adsorption processes.
Here, MOFs are a new class of porous materials with very interesting adsorption properties because of their ultra-high surface areas of up to 7000 m2/g, tailorable surface chemistry, and pore sizes. Previously, different types of MOFs have been identified for use in biogas upgrading (CO2 separation) and for methane storage in ANG, for example, Zeolitic Imidazolate Framework-8 (ZIF-8) and a MOF developed at Hong Kong University of Science and Technology (HKUST-1). Despite some excellent intrinsic properties of MOF raw materials, it is still a challenge to shape the raw powders into useful geometries, for example, granules, honeycombs, or extrudates, which allow their use in gas adsorption processes, do not degrade the original properties and enable good mass and heat transfer.
This thesis investigated different techniques (such as electrospinning, in-situ growth and phase conversion methods) to synthesize and structure well-known MOF materials (ZIF-8 and HKUST-1) and polyacrylonitrile (PAN) into porous MOF-polymer composite nanofibers for CO2 separation and CH4 storage. PAN nanofiber mats can be decorated with ZIF-8 or HKUST-1 crystallites by a phase conversion process, which includes the pretreatment of fibers, coating with zinc hydroxide or copper hydroxide, and dipping in a solution of the organic ligands (2-methylimidazole; 1,3,5-benzene tricarboxylic acid). The ZIF-8/PAN composite nanofibers showed a high BET surface area of 888 m2/g, and CO2 adsorption isotherms revealed gravimetric CO2 uptake capacities of 130 mg (CO2) /g (ZIF-8/PAN nanofibers) (at 298 K and 40 bar) with stable cyclic adsorption performance. X-ray diffraction (XRD) patterns and Scanning electron microscopy (SEM) showed ZIF-8 nanocrystals with sizes between 20 and 75 nm evenly distributed on the surface of the PAN nanofibers. Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and carbon-13 nuclear magnetic resonance (13C NMR) investigations indicated electrostatic interactions and hydrogen bonds between the ZIF-8 structure and the PAN nanofibers.
In-situ high-pressure Differential Scanning Calorimetry (HP-DSC) was used to measure the heat flow resulting from the adsorption of CH4 on the ZIF-8/PAN nanofibers at 298 K and 40 bar, and a total heat of adsorption of -15.51 J/g was calculated. Considering the mass loading of 40.7 wt.% ZIF-8 (neglecting the minor adsorption capacity of PAN nanofibers), the total heat of CH4 adsorption on ZIF-8 crystallites was -38.1 J/g (in accordance with -37.2 J/g of the adsorption of CH4 on the ZIF-8 nanopowders reported from Clausius-Clapeyron approximations). The experimental results were supplied as input to build a three-dimensional cell model to simulate the spatial and temporal heat transport in ZIF-8/PAN composite during the CH4 adsorption process. Such DSC measurements at high pressure can be helpful in future work to support modeling and predict the heat transfer in adsorbents and ANG tanks. Moreover, similar HP-DSC measurements were used to measure the heat flow resulting from the adsorption of CH4 on HKUST-1/PAN nanofibers and HKUST-1 nanopowders at 298 K and 40 bar. Benefits to good dispersibility of MOF particles in nanofiber structure, the exothermic adsorption time of HKUST-1/PAN nanofibers was shortened by 17% than that of HKUST-1 nanopowders.
The synthesis and structuring part of this thesis demonstrated an interesting method to prepare flexible MOF-polymer composites with high open porosities. Results of adsorption tests showed that these types of MOF-polymer composite nanofibers present high gravimetric gas uptake but lead to low volumetric gas uptake for applications as structured adsorbents in CO2 removal from biogas via pressure swing adsorption (PSA) or in ANG-based CH4 storage. However, with their extremely high open porosity, large surface area, and low pressure drop, such MOF-polymer composite nanofibers have enormous potential for use in related applications, such as gas and air filters for capturing and removal of toxic gases and particles (including viruses).
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages131
Publication statusPublished - 2022

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